The present invention relates to a fine copper wire for electronic instruments and a method of manufacturing the same. The fine wire is suitable, in particular, as a bonding wire used for the manufacture of semiconductors.
In the manufacture of semiconductors such as transistor, IC, etc., a fine wire of Au, Al or alloy thereof with a wire diameter of 15 to 100 .mu.m or thereunder is used between the electrode pad of Al connected to circuit elements and the lead of semiconductor for the connection of circuit elements on Si chip to external power source and for the giving and taking of external informations.
The fine wire of Al or Al alloy has an advantage in that the bonding to the electrode pad can be made with the same kind of metal, and is low in cost. However, since the ball bond is generally difficult from the reasons of being apt to be oxidized etc., the wedge bond using supersonic wave, which is poor in the productivity, has been adoped. Further, the fine wire of Al or Al alloy is poor in the corrosion resistance and, in particular, tends to cause the corrosion in the case of resin sealing type semiconductor due to the permeation of moisture. Therefore, it is used exclusively for the gas-tight sealing type semiconductor.
On the other hand, the fine wire of Au is not only excellent in the corrosion resistance, but also good in the bondability to Al. Therefore, it has an advantage to be utilized for the ball bonding, which is high in the productivity, and is sued widely with the resin sealing type semiconductor as a leader.
However, the material Au is not only conspicuously high in cost, but also liable to produce a fragile layer of intermetallic compounds, which is called purple plague, with Al or Al alloy of electrode pad under an environment of high temperature. As a result, the deterioration of bonding strength occurs with the lapse of time and the increase in the resistance of bonded area or, in a serious case, the break-down of wire comes to occur.
Moreover, the fine wire of Au does not adhere to the interface of resin and the gaps inevitably produced become the paths of moisture to allow it to permeate easily to the portion of electrode pad. Consequently, Al is corroded by Galvanic pair formed between Au and Al and the increase in the resistance of connection and the break-down of wire sometimes occur similarly to said purple plague. In particular, accompanying with the high integration and getting larger in size of chip and the miniaturization and pin-multiplication of package, a drastic lowering in the reliability resulting from the high temperature due to an increase in the density of heat generation, the shortening in the path of water vapor permeation and the increase in the probability of water vapor permeation is feared.
For there reasons, the development of fine wire which can be substituted for the fine wire of Au and the characteristics of which are also equal to or higher than those of fine wire of Au is desired. Although a fine wire of Cu is proposed for this purpose, the deformability thereof is inferior to that of Au and such problems that the cracks are generated under the electrode pad and the bonding between electrode pad and Al is insufficient are caused. In particular, with high-integration IC, fragile insulation layers of SiO.sub.2 etc. exist most frequently under the electrode pad and the development of fine wire of Cu, which has the deformability of equal to or higher than that of Au, is desired.
With regard to the fine wire of Cu, a fine wire of Cu containing 0.1 to 2% (1000 to 2000 ppm) of Be, Sn, Zn, Zr, Cr or Fe in Cu is proposed in Japanese Unexamined Patent Publication No. Sho 57-149744. This is excellent in the mechanical strength at high temperature, but it is far hard compared with the fine wire of Au. For this reason, excessive mechanical stress is applied to the semiconductor chip or the circuit substrate upon bonding resulting in the occurrence of damages of chip such as the cracking, cratering, etc.
This fact is particularly remarkable in the ball bonding and the ball formed is joined to the electrode pad on the semiconductor chip by means of the thermo-sonic method combined with thermal pressing and ultrasonic wave energy. The conditions at this time are temperature of 150.degree. to 350.degree. C., load of 10 to 200 g, output of ultrasonic wave of 10 to 200 mW and time of 10 to 100 msec. And, the hard ball requires always high bonding conditions, which bring about not only the lowering in the reliability and the lowering in the productivity, but also the fatal defect causing said cracking.
Furthermore, with regard to the fine wire of Cu, a fine wire of Cu containing 5 to 20000 ppm of rare-earth elements is proposed in Japanese Unexamined Patent Publication No. Sho 59-139662, a fine wire of Cu containing 10 to 20000 ppm of rate-earth elements, alkaline earth elements, Pd, Pt, Au, Bi, Te, Ge, Si, In, Al, B, Cd, Ti, Hf, V, Nb, Tl or Y, in particular, 100 to 10000 ppm of Y, rare-earth elements or Hf is proposed in Japanese Unexamined Patent Publication No. 61-20693, a fine wire of Cu containing 5 to 50 ppm of Ti, Cr or Fe or 10 to 100 ppm of Ni or Co is proposed in Japanese Unexamined Patent Publication No. Sho 61-99645, and a fine wire of Cu containing 5 to 50 ppm of Zr or Nb or 10 to 100 ppm of Pd, Ag, In or Sn is proposed in Japanese Unexamined Patent Publication No. Sho 61-99646. All of these are Cu alloys having relatively high concentration, though not so much as the fine wire of Cu proposed in Japanese Unexamined Patent Publication No. Sho 57-149744 aforementioned, and thus there is a problem in the hardness of ball. According to the experimental experiences by the inventors, the hardness of the ball of Cu is influenced significantly by the purity, in particular, the elements that form the solid solution and the purer the softer. In this respect, RRR (Residual Resistivity Ratio) having spread as a practical index of the purity of Cu is reasonable as a measure of bondability. The fine wires of Cu proposed in said Patent Publications have RRR of 200 to 500, which is larger than 100 to 300 of tough pitch copper or oxygen-free copper used ordinarily for the electrical use, nevertheless is an extent being much the same. This fact is insufficient in the industrial use, for which the bonding of LSI or VLSI with particularly high integration is made at high speed.
As a result of diligent investigations in view of this situation, a fine copper wire for electronic instruments suitable for the bonding wire and a method of manufacturing therefor have been developed by the invention.